In high-speed thermal printing, the individual print head element temperature must be accurately controlled to assure print quality. This temperature is determined by the energy electrically deposited into the heating element for each cycle, the cycle time, the printhead substrate temperature, and the ambient temperature.
Individual dot temperature is a time-axis phenomena wherein dot elements which undergo repetitive printing at print rates that exceed the thermal response of the print system experience self-heating effects. Dot elements that are seldom reprinted remain at the ambient temperature of the print head array.
Print head substrate temperature is a function of both printing and ambient temperature. Relative to printing, areas on the print head where groups of contiguous dot elements are continually reprinted (e.g., block graphics) can cause an elevation of the print head substrate temperature. Elevating the substrate temperature reduces its heat sinking capability which affects the time-axis response of individual dot temperature and can also endanger IC driver chips.
Significant ambient temperature variations can also cause these problems since, over time, the substrate temperature will vary as a function of the ambient temperature. Variations in the substrate temperature of the print head can affect the thermal response characteristics of the printhead as well as the differential energy required to produce thermal transfer.